This invention relates to surge protective devices (SPD's) and, more particularly, to a low impedance, or low-Z cable for use to connect SPD's in power line applications.
A surge protective device, or SPD, is used in power distribution network applications to protect loads connected to the network from high voltage surges or transients. Examples of the types of installation in which SPD's are used include centrifugal fire pumps, HVAC systems, computerized numerical control (CNC) machines, PLC's, and uninterruptible power supplies (UPS) for computer systems. SPD's use a variety of protection technologies. These include zener and selenium diodes, metal-oxide and silicon carbide varistors, and crowbar devices such as triggered and untriggered spark gaps.
In use, a SPD is connected across two feeder lines of the power distribution network. In a three-phase distribution system this would be one of the phase lines, and neutral; or, between phases, phases-to-ground and neutral-to-ground. An SPD can be connected on either the service side or load side of a service distribution buss. It can also be located on branch service busses and at distribution panels. Often, SPD units consist of a collection of SPD modules parallel wired to terminal blocks, as well as to disconnects inside a unit. When a voltage surge propagates down the conductor lines, it is sensed by the SPD. If the surge voltage exceeds the threshold level of the SPD, the SPD then presents a short-circuit across the conductors until the surge level falls back below the threshold. The downstream loads, especially those of relatively high impedance, are thus protected from the surge voltage.
It will be appreciated that in an ideal network, the SPD would present a perfect short-circuit in front of the loads, and would divert all of the current back to the source. However, because most configurations are less than ideal, the SPD is not necessarily exposed to all of the transient voltage. This is because while power distribution systems are designed to efficiently transmit 60 Hz power, they are not designed to transmit fast transient surges; i.e., voltage spikes of about 10 microsecond (10.sup.-6 sec.) or faster rise time. Consequently, some of the surge voltage is "let through" to the loads. Subjecting the loads to these high voltage transients is harmful to them. One culprit in this regard is the wiring or cabling used to connect the SPD in parallel with the network conductors. Conventionally, this cable is a shielded twin conductor cable. Shielded twin cables include two parallel conductors of radius r embedded in an insulator material with a distance w between the longitudinal axis of the conductors. A shield (typically conduit) encloses the conductors and insulator. The transient voltage drop across the wiring used in these shielded twin cable applications is sufficiently high that the SPD is not exposed to the full amplitude of a voltage surge. Accordingly, either the SPD is not switched into operation; or if it is, switching occurs at a higher transient voltage level than that to which the device is ultimately designed. Having available a lower impedance cable specifically for use in these configurations would allow the SPD's to be more effective in protecting downstream loads from exposure to excessively high voltages.